I Field of the Invention
The invention relates to unblown polymer modified asphalt compositions that exhibit improved performance grade specifications for high temperature properties. The invention further relates to a method of improving ethylene-vinyl acetate copolymer efficiency in polymer-modified asphalts.
II. Description of the Prior Art
It is known that adding polymer to asphalt improves the high temperature performance grade (PG) of paving asphalt cements as defined under the test methods established by the Strategic Highway Research Program (SHRP). Commonly used polymers include ethylene-vinyl acetate (EVA) copolymers and styrene-butadiene-styrene triblock (SBS) copolymer. These polymers are blended into the asphalt using high shear mix conditions to ensure proper dispersion of the polymer. Of the polymers used, SBS polymers are preferred because of their compatibility with a large number of asphalts. In addition, they can be crosslinked using vulcanizing agents such as sulfur.
Blending polymers with paving asphalts produces a change in the viscoelastic behavior of the asphalt. The change in viscoelastic properties is attributed to an increase in viscosity and elasticity and the formation of a network structure. The change in viscoelastic properties is related to the amount of polymer added, with network formation occurring above some finite polymer concentration. The amount of polymer needed can be reduced by adding a vulcanizing (crosslinking) agent such as sulfur. The crosslinking reaction allows the network structure to form at a lower polymer concentration. This improves polymer efficiency, reducing the amount of polymer needed to make a specific grade of asphalt cement. Unsaturated polymers such as SBS copolymers are easily crosslinked, but EVA copolymers which lack the requisite unsaturation are not.
Crosslinking enhances the efficiency of the polymer so that a lower concentration can give the desired improvement in the SHRP high temperature performance grade. The crosslinking is believed to occur through the reaction of sulfur with residual double bonds of the butadiene portion of the polymer. Unfortunately, the unsaturation in the SBS copolymer makes it more susceptible to degradation by atmospheric oxygen. EVA copolymers do not contain residual unsaturation and are not so easily degraded; however, the lack of unsaturation makes it difficult to crosslink these polymers.
U.S. Pat. No. 4,454,269 to Goodrich teaches the use of phosphoric acid and P.sub.2 O.sub.5 as a catalyst for air-blowing of asphalt/EVA copolymer blends. The purpose of the air blowing step is to improve the compatibility of the EVA copolymer. Simply blending the EVA copolymer into an airblown asphalt leads to incompatibility of the polymer. In contrast, the present invention relates to improving high temperature properties for unblown asphalts.